This article is about fundamental and derived units pdf type of unit of measure. This article needs additional citations for verification.
Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. A base quantity is one of a conventionally chosen subset of physical quantities, where no quantity in the subset can be expressed in terms of the others. In the language of measurement, quantities are quantifiable aspects of the world, such as time, distance, velocity, mass, temperature, energy, and weight, and units are used to describe their magnitude or quantity.
These relationships are discussed in dimensional analysis. In the International System of Units, there are seven base units: kilogram, metre, candela, second, ampere, kelvin, and mole. There are other relationships between physical quantities that can be expressed by means of fundamental constants, and to some extent it is an arbitrary decision whether to retain the fundamental constant as a quantity with dimensions or simply to define it as unity or a fixed dimensionless number, and reduce the number of explicit fundamental constants by one.
For instance, time and distance are related to each other by the speed of light, c, which is a fundamental constant. It is possible to use this relationship to eliminate either the unit of time or that of distance.
A similar choice can be applied to the vacuum permittivity or permittivity of free space, ε0. One could then eliminate the kilogram by setting ħ to a dimensionless number.